I recently had a young lady who weighed 130kg come to us for an AFR to be done in a 4 seat Cessna belonging to her family and the limit was 400 lb (from memory?) for the front row. She brought with her a letter sent to her from CASA requiring that a CFI/ATO/FOI had to do a report to say that she could do up the seatbelt, exit the aircraft and have full and free movement of the controls etc and CASA wanted to see the w+b for her AFR returned with the report. She had to lose a few kilos before I could put her with my lightest 60kg instructor for her AFR. Gave her a great incentive as it had to be done before her temporary medical extension ran out and she achieved that weight loss and more.
Happily she now has her AFR and medical and is getting back into flying professionally as well as getting back into better health.
Thing was she didn't look like she weighed nearly as much as that (and many people do weigh a lot more than they look as if they do so perhaps your instructor thus far was unaware of your weight tho that is not really an excuse - you have to do w+b for the GFPT after all). So the CASA requirement in this case must have been initiated by the medical people and passed on to the licensing people.